CABLE PROPERTIES OF CULTURED HIPPOCAMPAL-NEURONS DETERMINED FROM SUCROSE-EVOKED MINIATURE EPSCS

1. The passive cable properties of rat hippocampal neurons in dissocia ted culture were studied using focal application of hypertonic solutio n to locally elicit miniature excitatory postsynaptic currents (mEPSCs ) on the soma and dendrites. Neurons were filled with Lucifer yellow a nd portions of their dendritic trees were measured. 2. The average mEP SC measured at the soma appeared smaller and slower as the site of suc rose application was made more distal. Normalizing to a 1-mu m diam de ndrite, the mean mEPSC peak amplitude and charge was reduced e-fold in 170 and 1,000 mu m, respectively, and the mean mEPSC decay time const ant was increased e-fold in 150 mu m. However, for any particular sucr ose site, individual mEPSCs varied widely in their amplitudes and time courses. Plots of individual peak amplitudes versus half-width or ris e time showed much overlap for mEPSCs originating from sites as much a s 100 mu m apart. This suggests that use of such plots to estimate the electrotonic location of synaptic currents is highly prone to error. 3. Averaged mEPSCs recorded when applying sucrose at the soma were poo rly fitted by an alpha function but were well-described by an equation of the form m(x)h, where m incorporates a rise-time constant tau(1) a nd h a decay time constant tau(2). Averaged fits to mean mEPSCs elicit ed at the somas of five cells gave (mean +/- SE): peak conductance = 8 32 +/- 126 pS, tau(1) = 0.29 +/- 0.06 ms, tau(2) = 3.03 +/- 0.24 ms, x = 4.7 +/- 0.7. 4. For three cells, the entire dendritic branch to whi ch sucrose was applied was measured and used to construct a passive ca ble model. The specific membrane resistance (R(m)) and intracellular r esistivity (R(i)) were varied systematically in the model (assuming me mbrane capacitance C-m = 1 mu F/cm(2)) to search for the best agreemen t between the mean mEPSCs and the model. Optimal R(m) was found to lie in the range 20-30 k Omega cm(2), R(1) in the range 100-200 Omega cm. 5. These results confirm those obtained by other methods and emphasiz e the considerable cable filtering of fast electrical events in cultur ed hippocampal neurons.